1. Field of the Invention
The present invention relates to a filter, such as may be used to attenuate unwanted radiation and transmit desired radiation. More particularly, the invention relates to the use of the filter in a lithographic projection apparatus comprising:
a radiation system for supplying a projection beam of radiation;
a first object table for holding a mask;
a second object table for holding a substrate; and
a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate.
2. Background of the Related Art
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system generally comprises an illumination system which may also include elements operating according to any of these designs for directing, shaping or controlling the projection beam of radiation, and such elements may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. In addition, the first and second object tables may be referred to as the xe2x80x9cmask tablexe2x80x9d and the xe2x80x9csubstrate tablexe2x80x9d, respectively. Further, the lithographic apparatus may be of a type having two or more mask tables and/or two or more substrate tables. In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more stages while one or more other stages are being used for exposures. Twin stage lithographic apparatus are described, for example, in International Patent Applications WO 98/28665 and WO 98/40791, incorporated herein by reference.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (comprising one or more dies) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions which are successively irradiated via the reticle, one at a time. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire reticle pattern onto the target portion at once, such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally less than 1), the speed V at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205, incorporated herein by reference.
Problems with the prior art include that, in the semiconductor manufacturing industry, there is increasing demand for imaging ever smaller features at increasing densities. This is necessitating the use of shorter wavelength radiation, for example ultraviolet light with a wavelength of 157 nm or 126 nm. However, there is a problem that, as the wavelength decreases, the radiation becomes more energetic and there is an increasing tendency for optical components (including the mask) in the lithographic apparatus to show some fluorescence effects. The particular fluorescence light observed depends on the color center or centers in the material of the optical components. In the case of fused silica and CaF2, the emission of fluorescence light with marked colors can be observed, such as red, green, yellow and blue visible light. Similarly the materials used for the constituent layers of multi-layer coatings, e.g. a multi-layer anti reflex coating, or the materials used for single layer coatings, e.g. single-layer anti reflex coatings, on the optical components or on the mask may exhibit fluorescence effects. Also any contaminant present in the path of the projection beam of radiation may exhibit fluorescence effects. The fluorescence light can be actinic, i.e. the resist can be sensitive to this light, particularly when it is at the short wavelength or blue end of the spectrum, e.g. when its wavelength spectrum is between 350 nm and 550 nm. Thus the fluorescence light can cause unwanted exposure of the resist, which can cause a general degradation of the contrast of the image pattern since the fluorescence light is generally emitted in all directions and so constitutes a background dose to the resist. This can be detrimental to the quality of the exposure and can affect the process parameters.
MgF2 and BaF can also be used for the lens material in the lithography apparatus, but these materials can also have some fluorescence effect.
An object of the present invention is to avoid or alleviate the above problems.
According to the present invention there is provided a lithographic projection apparatus for imaging of a mask pattern in a mask onto a substrate which is at least partially covered by a layer of energy sensitive material (resist), the apparatus comprising:
a radiation system for supplying a projection beam of electromagnetic radiation with a wavelength less than 160 nm;
a first object table for holding a mask;
a second object table for holding a substrate;
a projection system for imaging irradiated portions of the mask onto target portions of the substrate;
characterised by further comprising:
a filter, located in the projection beam path, for attenuating fluorescence radiation with a wavelength longer than that of the projection beam.
An apparatus according to the invention, comprising a filter, can enable the radiation (such as 157 nm or 126 nm light) for defining the projected image to be transmitted, whilst the filter attenuates unwanted fluorescence light by reflecting and/or absorbing it.
The filter preferably comprises a plurality of layers, each layer having a refractive index, wherein the refractive index alternates between a relatively high value and a relatively low value or vice versa between successive layers.
The materials of different refractive index and the number of layers can be selected to define the transmission and attenuation characteristics of the filter.
The filter can comprise either a coating formed on a dedicated carrier substrate, preferably with substantially planar parallel surfaces, or a coating formed on another optical component. The latter possibility makes it very compact and straightforward to incorporate the filter in a given optical system.
The filter can also comprise two or more carriers placed in series in the projection beam, each carrier provided with a coating transmitting the desired radiation and attenuating a particular constituent part of the wavelength spectrum of the unwanted fluorescence radiation. This approach allows relaxed design specifications for each constituent filter in the series. A single filter or a series of filters, as described above, may hereinafter be referred to as the xe2x80x9cfilterxe2x80x9d.
According to a further aspect of the invention there is provided a method of manufacturing a device using a lithographic projection apparatus comprising:
a radiation system for supplying a projection beam of electromagnetic radiation with a wavelength less than 160 nm;
a first object table for holding a mask;
a second object table for holding a substrate; and
a projection system for imaging irradiated portions of the mask onto target portions of the substrate; the method comprising the steps of:
providing a mask bearing a pattern to said first object table;
providing a substrate provided with a radiation-sensitive layer to said second object table;
irradiating portions of the mask and imaging said irradiated portions of the mask onto said target portions of said substrate; characterised by the step of:
employing a filter, located in the projection beam path, to attenuate fluorescence radiation with a wavelength longer than that of the projection beam.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallisation, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.